1. Field of the Invention
The invention relates to a traffic simulator for testing exchanges, which simulator imitates the behaviour of subscribers and/or lines connected to the exchange by producing random variables such as call distances, call durations and reaction times, controlling the test run of a program-controlled control arrangement to which the subscribers imitations and/or line imitations are supplied, and checking the reaction of the exchange and/or mixing the technical statistic parameters of the exchange.
2. Description of the Related Art
Computer-controlled exchanges are generally formed by one or a plurality of single or multi-stage switching networks, a stored program central control and/or a plurality of decentralized control units and adaptor units, which connect the control units to the switching networks. In addition, such an exchange includes transmitters and receivers for signalling data, peripheral control units and further peripheral devices. The exchange receives and responds to signalling and control pulse transmitted to it to the exterior, that is to say from its terminals, for example by subscriber lines, or other connection lines (see FIG. 1).
By means of a traffic simulator a model, i.e. a functional imitation of the exterior (peripheral) network is made and the behaviour of the exchange in realistic traffic conditions is recorded. The external events, such as, for example, accessing subscriber's lines or connection lines, dialling, and lifting the handset by the called subscriber, which are created outside the exchange, are then recorded as event patterns which represent the sequence of events and the times at which such events start. These sequences of events are produced in the traffic simulator, their spectra varying from simple event patterns which are fixed in a controlled manner, to random event patterns. In this situation, the event patterns are determined by the event sequences of all call types prevailing in a simulation. Because of the fact that the external events can be described by event patterns, substantially all the details of a real call can be taken into account.
With such a traffic simulator it is for example, possible to describe and simulate call set-up stages such as lifting the hand set by the calling subscriber, accessing a junctor circuit, waiting for the dialling tone, the dialling phase, signalling, through-connection, calling the desired subscriber, taking the hand-set from the cradle by the called subscriber, release etc. This composite procedure of the exchange process is characterized in that continuation of the process requires new data which are contributed by exterior data sources and at random instants. These new data which occur during the exchange process and are used as input data for continuing the process, are stored in buffer stores. The dialling phase is, for example, characterized in that it is assembled from a plurality of pulses of a predetermined duration (digit), the time intervals between the pulses (digits) following, for example, a predetermined distribution.
The representation of all types of incomplete calls, such as, for example calls with incomplete dialling, the called subscriber is busy, no answer etc. or calls from and to specific terminals such as, for example, private branch exchanges, connections to push-button stations, abbreviated dialling, visitor circuit and further performance characteristics of the exchange station can be simulated by the different event samples. In these situations the event samples do not only imitate all the individual events, but also reflect the influence of the correlation between the occurrence of an event and its arrival, in time, in the central exchange. The time spacings between the events of each type of call can be selected in accordance with special distribution functions which are valid for the use to which the central exchanges are put. Selecting the distribution function is based on statistical tests, the underlying test results being obtained by observations and evaluation of the telephone traffic.
Using the traffic simulation arrangement it must be possible to effect checking of the system behaviour of any central exchange, that is to say a telephone or data exchange, for the public or private (branch) section, as regards their performance and functional capability during the set-up and cancelling of an individual connection up to, for example, one thousand parallel active television and/or data connections, by simulating the behaviour of the subscribers and lines connected thereto. Such a traffic simulation arrangement can then be utilized for checking the efficiency of the central exchange as regards traffic handling and its function as an exchange, the determination of the soft-ware efficiency under load conditions, checking overload protection strategies, investigating the behaviour of one or a plurality of control computers (control units) for brief traffic peaks etc.
During the development and production of a prototype of a central exchange (telephone or data exchange) the establishment of specific technical exchange parameters (test values), for example dialling tone delay, ringing signal delay etc.) is necessary, to enable testing of the designed prototype for its system properties and to be able to determine the effects of design modifications on the unchanged portions of the overall system. With the aid of these technical exchange parameters (test values) it is then possible to prove, when the exchange is handed over to the customer, that the requirements this customer has made as regards the system properties (which are laid down in a standard specification) are respected.
West German Pat. No. DE-PS 32 12 019 discloses a method for an arrangement for traffic simultaion in telephone exchanges, by which the effect of the behaviour of a subscriber on the telephone exchange can be simulated by automatically controlled subscriber imitations. With the aid of the subscriber imitations, controlled by a control computer unit, connections are set-up and cancelled and the reaction of the relevant central exchange is tested. The overall procedure of a connection to be established between two subscribers imitations, is laid down in a store as an an empirically determined command sequence and has intervals at instants which, as is customary, depend on the reaction of the subscriber. To generate a random subscribers behaviour during a predetermined (programmed) functional procedure, intervals are produced, statistically distributed, by means of random generators. During testing of the proper operation of mass-produced central exchanges, each measured parameter is compared with a standard sample (for example obtained by measurements taken on the prototype). For deviations between measured parameters of the actual central exchanges and the specific parameters of standard samples, a corresponding error report is transmitted and the detected failure is eliminated. In doing so, it is possible to check whether the required system parameters and system properties as regards the set-up and cancellation of an individual connection are respected.
In the method disclosed in patent No. DE-PS 32 12 019 subscribers imitations are used to test the hardware and software of the central exchange. The subscribers imitations generate trigger pulses intended for the exchange system, either individually or in groups, and the corresponding system reactions are compared with a standard sample. The trigger pulses assigned to each individual subscriber are determined on the basis of the desired functional procedure (test procedure) in their chronological sequence and at their instant of occurrence. During the comparison with the standard sample, the error report in the case of malfunctioning is put on record (and optionally the test is ended); when there is agreement between the measured parameters and the standard sample, the trigger pulses assigned to a subscriber or a line, respectively, are periodically repeated after exactly the same test procedure.
In that method all the trigger pulses used during the test procedure are predetermined before the beginning of the test as regards their types and instants of appearance. For example, the digits of the subscriber to be dialled (B-Tln), determining that only the called subscriber (A-Tln) or only the B-Tln terminates the call; determining that the speech connection can be effected immediately after a recognised successful call set-up; determining that the time spacing between two event instants satisfies to a limited extent statistical laws for only a portion of all the possible time intervals; are stored. Prior art test arrangements, such as, for example, the call simulator disclosed in West German Pat. No. DT-AS 24 20 773 or the test arrangement disclosed in West German Pat. No. DE-PS 32 15 672, relate only to a physical supervision of the connections to be set-up.
The known test methods (Nos. DE-PS 32 12 019, DT-AS 24 20 773, DE-PS 32 15 672) have the disadvantage that they can only test the physical and logic system properties for a limited number of simultaneously existing connections. However, these system properties (reactions) change when the central exchange is loaded by a maximum system configuration with a nominal load or is overloaded, and due to real subscriber behaviour.
Nominal load must here be understood to mean the maximum possible system configuration (that is to say a maximum line configuragion and/or a maximum number of connected subscribers) and an offered traffic intensity of typically a=0.1 Erl per subscriber and a=0.8 Erl per line. These traffic intensity values change significantly in the course of a day in the so-called main traffic hours or in special periods (in the event of catastrophies). Additionally, brief overload peaks which are critical for the functional reliability of the system may occur. In addition to an increased amount of traffic offered and the consequently increased arrival rate also the what is commonly referred to as the subscriber behaviour changes.
Subscriber behaviour must here be understood to mean that the subscriber becomes nervous or impatient when the connection is not set-up as anticipated by him, which becomes manifest by, for example, an increased number of redialling operations, or that during the call set-up decisions which deviate from the normal rule are made (for example the dialling operation is prematurely terminated). These redialling operations are trigger pulses for the system without, relative to that individual call, a successful call connection can be ascertained.
These different effects as regards the traffic characteristics are taken account of in the prior art test method. The traffic flows thus produced can be represented in a model of a subscriber or a subscribers group of any size (imitation of the traffic circumstances) by the following applied traffic functions:
______________________________________ finite number infinite number of sources of sources ______________________________________ stationary .alpha./Tln .lambda. non-stationary .alpha.(t)/Tln .lambda.(t) ______________________________________ where .lambda. is the call arrival rate.